36
J . B. GURDON
an event which is sometimes accompanied by chromosome breakage.
When a transposable element such as Ds comes to lie next to a gene, the
gene becomes unstable and as a result tends to undergo stable changes
affecting its expression. These changes differ from mutations in the high
frequency of their occurrence, and in the specificity of the change undergone. Like mutations however, they are stable except when the particular controlling elements to which the gene responds are present. A
further peculiarity of Ds is that it will only cause instability at a nearby
gene locus if there is present somewhere in the same nucleus another
transposable element called Ac. The nature of the transposable elements
as well as of the changes they induce is quite unknown. Since McClintock
has found other systems of controlling elements in maize like the
Ac-Ds one, it is possible that such transposable elements may pervade
the genome and act so as to control the expression of genes in development. The main objection to this suggestion, as Brink (1958) has very
clearly pointed out, lies in the sporadic occurrence of the gene changes
promoted by the transposable elements. If these elements are generally
important in differentiation, they must consistently induce gene changes
in certain tissues at certain stages in development. Contrary to this
requirement, transposable elements may move to any part of the
chromosome set at irregular times in different cells. Furthermore cell
differentiation proceeds normally in cells from which these elements
have been lost. While these transposable elements may possibly have
an important function in cell activity, it seems most unlikely that they
play any part in promoting cell differentiation.
The last kind of stable nuclear change to be discussed here is macronuclear differentiation in ciliate Protozoa. Following normal conjugation in Paramecium most of the daughter micronuclei as well as the
macronucleus degenerate. The new macronucleus is formed from a
division product of the reconstituted micronucleus and increases in
size progressively during the subsequent divisions of the ex-conjugants.
After conjugation, the ex-conjugants as well as the daughter cells to
which they give rise are sexually immature so that they will not conjugate again for several divisions. Siegel (1961) has shown that after
heat-treatment during conjugation a new macronucleus is formed from
fragments of the old degenerating one and not from the newly formed
micronucleus as usual. Individuals which have undergone macronuclear
regeneration do not become sexually immature at conjugation as usual
but are already sexually mature just like the parent conjugants. Various
kinds of evidence show that sexual maturity is a character under the
control of the macronucleus or some associated component (e.g. Beale,
1954). I t can be concluded from this that the macronucleus normally
undergoes a kind of differentiation as a result of which it promotes
J . B. GURDON
an event which is sometimes accompanied by chromosome breakage.
When a transposable element such as Ds comes to lie next to a gene, the
gene becomes unstable and as a result tends to undergo stable changes
affecting its expression. These changes differ from mutations in the high
frequency of their occurrence, and in the specificity of the change undergone. Like mutations however, they are stable except when the particular controlling elements to which the gene responds are present. A
further peculiarity of Ds is that it will only cause instability at a nearby
gene locus if there is present somewhere in the same nucleus another
transposable element called Ac. The nature of the transposable elements
as well as of the changes they induce is quite unknown. Since McClintock
has found other systems of controlling elements in maize like the
Ac-Ds one, it is possible that such transposable elements may pervade
the genome and act so as to control the expression of genes in development. The main objection to this suggestion, as Brink (1958) has very
clearly pointed out, lies in the sporadic occurrence of the gene changes
promoted by the transposable elements. If these elements are generally
important in differentiation, they must consistently induce gene changes
in certain tissues at certain stages in development. Contrary to this
requirement, transposable elements may move to any part of the
chromosome set at irregular times in different cells. Furthermore cell
differentiation proceeds normally in cells from which these elements
have been lost. While these transposable elements may possibly have
an important function in cell activity, it seems most unlikely that they
play any part in promoting cell differentiation.
The last kind of stable nuclear change to be discussed here is macronuclear differentiation in ciliate Protozoa. Following normal conjugation in Paramecium most of the daughter micronuclei as well as the
macronucleus degenerate. The new macronucleus is formed from a
division product of the reconstituted micronucleus and increases in
size progressively during the subsequent divisions of the ex-conjugants.
After conjugation, the ex-conjugants as well as the daughter cells to
which they give rise are sexually immature so that they will not conjugate again for several divisions. Siegel (1961) has shown that after
heat-treatment during conjugation a new macronucleus is formed from
fragments of the old degenerating one and not from the newly formed
micronucleus as usual. Individuals which have undergone macronuclear
regeneration do not become sexually immature at conjugation as usual
but are already sexually mature just like the parent conjugants. Various
kinds of evidence show that sexual maturity is a character under the
control of the macronucleus or some associated component (e.g. Beale,
1954). I t can be concluded from this that the macronucleus normally
undergoes a kind of differentiation as a result of which it promotes